Video cassette recorders (VCRs) currently enjoy a wide popularity for recording and playing movies and other subject matter. The most common type of videocassette is a VHS format cassette commonly referred to as the "T-120", having approximately 246 meters of tape and allowing up to about 2, 4, or 6 hours of recording time, depending on the selected machine recording speed. It is often desirable, in playing or recording material on a VCR, to have a fairly accurate estimate of where on a tape a certain program is contained, and/or how much unrecorded tape remians on a given side of a videocassette, after a certain amount of recording has already taken place. Many VCRs are equipped with a tape-spool revolution counter which provides the operator with some information about tape position. However, these counters have been of limited use heretofore, since the machine-indicated tape-spool revolution values often do not reflect true tape spool revolution values, and actual tape-playing time may not be linearly related to the tape revolution values. It is known, for example, that the ratio of machine-indicated tape-spool revolution value/actual tape spool revolution value can vary from about 0.9 to 4.0 in VCRs currently in use. As a result, even where a revolution counter is available, operators of VCRs frequently are in doubt as to how much actual available recording time remains on a partially recorded videocassette or how long a program already recorded actually is. Lack of this information can result in waste of valuable videotape, as the operator will frequently use a new videocassette when he might have had space on one of the partially recorded tapes already in his possession. Thus, a device which could compute the available recording time remaining on a videocassette would be useful.
In addition, many operators, in indexing their videocassettes, prefer to show the length, in time, of a recorded program. Unless the time was noted at the time the recording was made, a tape would have to be "run through" at normal playing speed to verify it's actual length in time. Thus, a device which could compute the length in time of already recorded subject matter would also be of use.
Further, because of the wide variablility in machine tape-revolution counters, tape-revolution indexing data which applies to one machine may be of little use if the same tape is used with another machine, such as where a user exchanges a tape with another VCR user. As indicated above, the variation in machine-indicated tape-spool revolution value may be as much as fourfold among different VCR machines, and there is currently no industry standard which governs the relationship between digital counter readings and elapsed time or tape travel. Thus, a device which could enable the operator, after a calibration procedure, to cross index a borrowed videocassette to his particular machine would be useful.